Three-dimensional integrated composite surface structures

ABSTRACT

A surface structure including a skin ( 13 ) a build-up ( 18 ) and a backing ( 20 ). A skin is formed and a build-up layer is applied to the skin. In one embodiment, gas pressure is applied to form the skin in a single-sided die. A nozzle on a five-axis arm then applies resin to the skin in a selected pattern. The selected pattern may form a hexagonal “honeycomb” cell structure, for example, to support the skin and may transition to a columnar cell structure for better crush resistance. The hexagonal-to-columnar structure provides superior crush resistance in automotive body panels, for example, and improved solar energy concentration in solar energy applications.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is being filed concurrently with the followingU.S. patent applications: AN ALUMINUM-THERMOPLASTIC PANEL AND METHOD OFMANUFACTURE by Hillier (Atty. Docket No. 018981-000100); APPARATUS FORFABRICATING SURFACE STRUCTURES by Hillier (Atty. Docket No.018981-000300); and METHOD AND APPARATUS FOR FORMING SURFACE STRUCTURESFROM A SINGLE-SIDED MOLD by Hillier (Atty. Docket No. 08981-000400).

STATEMENT AS TO THE RIGHTS TO INVENTION MADE UNDER FEDERALLY SPONSOREDRESEARCH AND DEVELOPMENT BACKGROUND OF THE INVENTION

[0002] This invention is concerned with surface structures havingimproved structural characteristics over conventional shaped panels,such as may be used in the manufacture of vehicles or in otherapplications, such as aerospace, industrial applications, militaryapplications, and recreational products. The invention is directed atreplacing currently used materials in existing applications, as well asproviding new materials for use in new or existing applications.

[0003] Panels are used in a wide variety of applications, and come in awide variety of materials and shapes. Some panels are flat, and some arebent or stamped into a shape. For example, a body panel on an automobilemight be stamped from sheet steel or other alloy. In some cases, such apanel might be too weak to act as a structural component of theassembly, and is fastened to a frame or chassis. In other instances, thepanel may be sufficiently strong to provide structural support, or maybe welded or otherwise joined with other sheet steel parts to form anassembly that includes the panel portion for use in the final product.

[0004] Unfortunately, a stronger panel generally means a thicker panel.It takes more energy to form a panel into a given shape if the materialis thicker, and thicker material might limit the shapes that can beformed. Also, a thicker panel is heavier. Thus, simply making a thickerpanel to obtain strength adds to both the material costs and thefabrication costs, as well as the weight of the final product,particularly if a panel is made from steel sheet.

[0005] Thinner panel pieces are often shaped and then spot-welded into abox-like assembly. The various pieces support each other to make astructural element of desired strength and rigidity, which often weighsless than either a thin sheet attached to a frame, or a thick structuralpanel. However, thinner panels are not without problems, such as dentingand rust-through. Thin sheet steel dents relatively easily, even fromminor impacts, and might rust entirely through in a short time ifcorrosion protection fails.

[0006] Alternatives to making body panels from stamped steel sheet havebeen developed to overcome some of the limitations of steel panels. Oneapproach has been to fabricate panels, or even complete bodies, out ofre-enforced resin (so-called “fiber-glass”) composites. Fiber-glassparts are generally lighter than comparative steel parts, allow greaterchoice in the types of shapes that may be fabricated, and do not rust.Fiber-glass parts are typically attached to a frame, as they aretypically not structural, as they tend to fail under strain when insheet form, although sometimes a shape is built up or adhesivelyattached to provide mechanical strength. However, fiber-glass parts tendto crack or splinter on impact, rather than absorbing much energy fromthe impact, and are considered to be relatively fragile, and scratcheasily. Other re-enforced resin systems, such as “carbon-fiber” systemshave been developed to improve some of the shortcomings of fiber-glassparts, but are typically more difficult to work with, and moreexpensive. In some applications, fiber-glass or other composite partsare molded into a thick, structural part of complex (multiple curvedsurfaces) part.

[0007] Molded parts and stamped steel or other alloy parts both aretypically made in a process using a two-part mold or two-part stampingdie, respectively. Making two-part mold tooling is fairly straightforward, but the resulting sprues and seams must be trimmed from thepart before the part can be considered finished. Also, the material,which is typically injected into the mold in a fluid form, musttypically be left in the mold long enough to solidify, either by coolingor by chemical reaction, to solidify enough to retain the shape of themold upon removal. The mold dwell time can slow down the entirefabrication process, thus increasing costs.

[0008] Stamping steel or other alloys also requires substantial toolingcosts. A stamp and die are both precision tools that must match, andthat typically accept a particular material of a particular thickness.Changing the design of a stamped part is expensive and time consuming.Stamping steel sheet or other alloy has other problems that limit thetype of shapes that can be formed. For example, there is a certainmaximum depth, also known as aspect ratio, that a particular sheet canbe drawn to. Trying to stamp sheet into certain shapes can cause pullingand stretching of the sheet, particularly puckering, or webbing, inareas adjoining the seam of a curved area.

[0009] Therefore, a panel that may take a shape with complex curves thatis light, strong, dent resistant, and corrosion resistant is desirable,and a method for making such panels that is efficient and adaptable tovarious materials and shapes is further desirable.

SUMMARY OF THE INVENTION

[0010] A surface structure including a skin portion bonded to a build-upsection and a backing provides a light, strong, versatile structuralelement. In one embodiment, the build-up section includes cells, thecell walls being essentially perpendicular to a curved surface of theskin portion. In another embodiment, the build-up section includes cellsthat transition from a hexagonal configuration (cross section) to acolumnar (round) configuration. These embodiments of the invention canbe combined for further benefit.

[0011] In one instance, gas pressure is used to deform a skin preform toa desired shape and the build-up is applied, as a settable liquid,semi-liquid, or powder, for example, in a desired pattern to the formedskin. In another instance, the skin is applied as a liquid or powder toa single-sided mold and the build-up, of similar or different material,is then applied in the desired pattern. In yet another instance, acasting pattern is made that includes a skin section and at least thebuild-up section. The casting pattern is made from wax, for example,which is then used in a lost-wax process or other process to cast apreliminary surface structure, to which the backing may be applied.

[0012] These and other aspects of the invention will be betterunderstood by reference to the following detailed description inconnection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1A is a simplified cross section of a composite panelaccording to the present invention;

[0014]FIG. 1B is a simplified cross section of a portion of a surfacestructure with a graded filler build-up section;

[0015]FIG. 1C is a simplified cross section of flat surface structureaccording to one embodiment of the present invention;

[0016]FIG. 1D is a simplified cross section of a flat surface structurewith a hexto-round build-up cell, according to another embodiment of thepresent invention;

[0017]FIG. 2 is a simplified cross section of a curved surfacestructure, according to another embodiment of the present invention;

[0018]FIG. 3A is a simplified cross section of a skin preform in asingle-sided mold;

[0019]FIG. 3B is a simplified representation of build-up being appliedto a curved skin;

[0020]FIG. 3C is a simplified cross section of a skin and build-upsection in a single-sided mold with a backing preform;

[0021]FIG. 3D is a simplified cross section of a surface structure in asingle-sided mold after application of the backing to the build-upsection;

[0022]FIG. 4A is a simplified flow chart of a process for making asurface structure according to one embodiment of the present invention;

[0023]FIG. 4B is a simplified flow chart of a process for making asurface structure with an explosive-set skin;

[0024]FIG. 5A is a simplified illustration of a three-station apparatusfor fabricating surface structures;

[0025]FIG. 5B is a simplified flow chart of a three-station fabricationprocess; and

[0026]FIG. 6 is a simplified illustration of a system for formingsurface structures.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] A composite panel including a skin, a center build-up section,and a backing can be fabricated into a variety of shapes, includingshapes having complex curves, from a variety of materials. The compositepanel results in a surface structure with desirable and selectablecharacteristics, such as weight, stiffness, corrosion resistance, impactresistance, and cost. The skin can be plastic or metal, such asaluminum, titanium, or stainless steel, for example, and the center canbe thermoplastic resin, thermosetting resin, or combinations thereof,including resin-filler composites, as well as other materials, such asmetal. In one embodiment, a nozzle on a multiple-axis carriagedischarges heated thermoplastic resin to build up a polymer center in aparticular pattern on a shaped skin. The polymer center has sufficientstrength to serve as a die for setting, i.e., shaping, the backing,which may be aluminum sheet, for example. This allows the center to bedirectly bonded with the skin and the backing, and also results inconformal fidelity and integration of the skin, center, and backingwithout the use of intermediary adhesives.

[0028]FIG. 1A is a simplified cross section of a composite panel 10according to one embodiment of the present invention. A skin 12 isjoined to a center section 14, also known as a “build-up”, which isjoined to an encapsulating skin 16, or backing. The skin can be metalsheet, such as 6061-O condition aluminum, other aluminum, such as1100-O, 3003-O, 5052-O, and 7075-O, titanium, stainless steel, such astype 4300, plain steel, such as ASTM-A620, super-plastic alloy, polymer,such as a thermoplastic or thermosetting plastic, including ABS,polycarbonate, polyethylene, polyurethane, and type I or II PVC, orother material, such as a woven material, including materials woven fromceramic or glass. It is generally desirable that the skin material havegood tensile strength.

[0029] The build-up 14 is a polymer material that is built-up on theskin, rather than being adhesively attached to the skin with anintermediary adhesive layer. The build-up is applied to the skin whilethe skin is in condition for the build-up to bond to the skin, eitherthrough mechanical bonding, chemical bonding, or both. In the case of ametal or thermoplastic skin, the skin can be heated prior to applyingthe build-up material. In the case of a thermosetting skin, the skin canbe partially un-cured while the build-up material is applied, or treatedwith a surface activator, thus allowing the skin to intimately bond withthe build-up. Examples of thermoplastic materials include polyvinylchloride, polycarbonate, acrylic resins, acrylonitrile-butadiene-styrene(“ABS”), polyethlyene, and polypropylene. Examples of thermosettingmaterials include epoxies, vinyl esters, polyesters, urethanes,phenolics, and polyimides.

[0030] The build-up material is applied to the skin as by spraying,flame-spraying, extruding, or otherwise dispensing, and is generallyapplied in a fluid state, such as a liquid, although some materials,including composite materials, may be applied in a powder state, andthen processed to bond to the skin. Of course, not all build-upmaterials will be compatible with all skin materials, but severalexamples are provided below. In an alternative embodiment, a castingpattern is made, such as by dispensing liquid casting wax on a wax sheetin the mold to produce a casting pattern for a surface structure.

[0031] The backing 16 is typically aluminum sheet, polyamide film, orthe like, and is intimately bonded to the build-up 14. The backingpreferably has good tensile properties, and is at least slightlyconformable, so that intimate contact with the build-up can be obtained.The back-up material can be heated or treated with a surface sensitizer,or only partially cured, when the backing is applied to obtain goodchemical bonding between the build-up and the backing. Additionally,pressure is used to press the backing against the build-up to bring thetwo layers in contact. The pressure may be mechanically applied, such asby rolling, or may be hydrostatically applied, such as by gas pressure.The build-up has sufficient strength to support the applied pressure.During this process, the skin is externally supported, by a table ordie, for example, so that the composite surface structure does notdeform. The combination of the skin 12, build-up 14, and backing 16 forma box-like structure in which the skin and backing, separated by thebuild-up, provide tensile strength to resist deformation of thecomposite surface structure in various directions.

[0032]FIG. 1B is a simplified cross section of a portion of a surfacestructure 11 according to another embodiment of the present invention.The build-up layer 15 is graded to obtain a selected property. Forexample, the buildup layer may start as a single phase 17 (pure build-upmaterial), and then transition to a filled material 19. The fillingmaterial 21 may be gas bubbles, such as nitrogen that is entrained inthe stream of build-up material as it is applied to the skin,microballons, such as glass microballons, microshpheres, such asaluminum, glass, or ceramic microspheres, or gas bubbles may begenerated in the build-up material itself. For example, if polycarbonateis used as the build-up material, the single phase portion of thebuild-up layer can be applied at a temperature that liquefies thepolycarbonate, and the temperature can be raised when applying thefilled material to spontaneously cause bubbles to form and becomeentrapped in the polycarbonate, such as supplying the polycarbonate at atemperature of about 485° F. with the nitrogen at about 210° F. at apressure of about 4-5 psi.

[0033] Selected superior material properties may be achieved dependingon the desired characteristics of the surface structure 11. For example,entraining or generating gas in the filled material section can lightenor improve the thermal insulation properties of the surface structure. Areactive gas, such as oxygen, can be entrained in a build-up material,such as polyester, to improve the cure time. In a preferred embodiment,microspheres are mixed with the polymer or thermosets used in thebuild-up layer to provide compressive strength. Other solid forms, suchas fibers, particularly carbon fibers about 0.0003 inches in diameterand less than about 0.063 inches long, may be used alternatively or inaddition to the microspheres, but microspheres generally provide bettercompressive strengthening than fibers as the microspheres do not exhibitas much of a problem arising from inter-laminate shear. Generally, fillratios range between about 20-33 volume %, but other fill ratios may beappropriate depending on the base material and filler used Themicrospheres may be solid, such as spherical aluminum metal powder, ormay be hollow, such as zirconia or glass bubbles. The microspheres rangein diameter from approximately 0.0015-0.008 inches, and a single size ofmicrospheres may be used in the build-up layer, or microspheres ofvarious sizes may used for better packing density of the spheres.Additionally, spheres of various materials may be combined in a singlebuild-up layer.

[0034]FIG. 1C is a simplified cross section of a flat surface structurehaving a having a skin 13, such as sheet metal, a build-up section 18,and a backing 20. The build-up section 18 includes a first zone 31contacting the skin 13 essentially over the entire interface between theskin and the build-up section, and a second zone having walls 33 thatform box-like voids 35 in the build-up section. The backing 20 seals thevoids, the combination of which provides stiffness to the surfacestructure. The walls of the voids also provide “give” to the surfacestructure for improved dent resistance by transferring loading to globalbuckling and then post-catastrophic failure of the hexagonal cells tolocal buckling. In some embodiments the voids may be evacuated toprovide improved thermal insulation or mechanical properties, or may bepressurized with air, nitrogen, water, oil, or other fluid to improvethermal conductivity, impact resistance, or other mechanical properties.

[0035]FIG. 1D is a simplified cross section of a portion of a surfacestructure showing cells that transition from a hexagonal cross section37, or pattern, to a columnar (round) cross section 39. The hexagonalcell pattern contacts the skin 13 in a continuous fashion, that is, thewalls of the hexagonal cells provide support to the skin without gapsbetween cells, thus improving the strength of the surface structure,while the columnar cells provide superior crush resistance. Thehex-to-round cell structure combines these characteristics to result ina superior support for a skin.

[0036]FIG. 2 is a simplified cross section of a portion of a surfacestructure 22 with a curve. The curve is shown only in the plane ofintersection, but may also have components of curvature that are notco-planar with the plane of intersection, and the surface of the skincan define a number of curves, such as may be defined with a non-uniformrational B-spline (“NURBS”) surface. The skin 23 is 6061 aluminum sheetthat was originally about 0.060 inches thick., but thinner aluminumalloy sheet could be used, such as 0.009 inch thick, or steel sheetabout 0.010 inches thick, or thermoplastic sheet 0.0045 inches thick.The 6061 aluminum sheet is in the O condition, as that condition is lessprone to cracking when formed than aluminum sheet in the T6 condition.The build-up 24 is polycarbonate, for example, and is dispensed in aselected pattern on the skin from a computer-controlled 5-axis nozzle toa final thickness of approximately 0.090 inches The nozzle and skin areheated so that the polycarbonate may be easily dispensed through thenozzle and form a good chemical bond with the skin. The surface of theskin has been textured during a forming process that will be describedin further detail below.

[0037] The polycarbonate, which may contain fillers is dispensed on theskin in a pattern resulting in three zones. Some types of fillers, suchas carbon fibers, may be used to strengthen the composite resin, othertypes of fillers, such as microballons, may reduce the effectiveviscosity of the resin to aid in dispensing the resin, as well asstrengthen the resin or improve other properties, such as thermalinsulation. The first zone 25 is a sheet of polycarbonate contacting theskin over its entire surface. Complete contact with the skin providesgood adhesion and shear strength between the skin and the build-upsection 24, which is about 0.004 to 0.030 inches, preferably at least0.010 inches, thick. The second zone 26 is a region of hexagonal cells,or “honeycomb”. The hexagonal cells are formed by dispensing thepolycarbonate in a hexagonal pattern over the first zone sheet 25 inmultiple passes. As the polycarbonate cools and solidifies afterextrusion from the nozzle, the hexagonal cells are built up. The walls26 of the hexagonal cells are essentially perpendicular to the surface27 of the skin around the curve to provide good support to the skin.Other laminates with hexagonal cores are generally limited to flatsections, so that the cell walls are perpendicular to the surface, orare machined to fit curved surfaces, in which case the cell walls arenot always perpendicular to the surface. It is desirable to have thecell walls perpendicular to the surface to resist buckling upon impact.If desired, the hexagonal cells may be tapered to improve impactdeceleration, or may remain constant in cross section to support higherstatic loads.

[0038] The third zone is a region of columnar cells 28. The build-uptransitions from the hexagonal zone 26 to the columnar zone 28. Thecolumnar cells are less likely to collapse than similar hexagonal cells,while the hexagonal cells provide superior support to the skin thancolumnar cells, thus a build-up of superior properties is obtained bytransitioning from the hexagonal structure to the columnar structure sothat global loading is converted to localized loading, and criticaldeformation occurs on the backing, rather than the skin. The transitionfrom one zone to another is achieved by controlling the delivery rate,direction, and speed of the delivery nozzle.

[0039] Applying the build-up to the skin offers many advantages overconventional laminates. As discussed above, all of the cell walls areessentially perpendicular to the skin, even the curved portions of theskin. Also, the thickness of the cell walls may be adjusted along thelength of the cell by varying the dispensing rate, e.g. pressure ornozzle speed, to optimize characteristics of the surface structure forvarious combinations of materials and configurations. The celldimensions, or pitch, can also be varied, to provide more cells in areasrequiring greater support, for example. The cross-cell dimension forhexagonal cells is generally between about 0.125-1.00 inches, and thecells typically have an aspect ratio of about 4.

[0040] A backing 29 of 6061 type O aluminum approximately 0.010 inchesthick is formed over the build-up 24. The backing is applied to thebuildup while the build-up is still warm enough to deform and bond tothe backing, but strong enough to provide at least about 70% of theultimate (cold) compressive strength of the build-up. Gas pressure isused to press the backing against and into the build-up, conforming thebacking to the surface of the buildup and intimately bonding the backingto the build-up to achieve a unitary surface structure 22 comprising theskin 23, build-up 24, and backing 29. The above structured build-up, orother structured build-ups, can be applied to flat panels, as well. Inalternative embodiments, metal is flame-sprayed from a nozzle to formthe build-up section on a formed skin. Higher aspect ratios may beobtained if the nozzle is used to flame-spray a skin section on thesingle-sided mold prior to forming the build-up section. Similarly, aplastic skin section may be dispensed onto a single-sided mold prior toforming a build-up section on the skin. The material used for the skinand the build-up may be the same, or different, and a single nozzle ormultiple nozzles may be used. When the skin is dispensed on the mold,differential thermal expansion/contraction between the build-up materialand the mold material can be used to assist in removal of the build-upmaterial.

[0041]FIGS. 3A to 3D are simplified cross sections illustrating aprocess according to the present invention. FIG. 3A shows a skin preform30, in this case a sheet of 6061 O-type aluminum 0.010 inches thick, inrelation to a single-sided die 32. The single-sided die 32 is shown witha simple curve 34 for purposes of illustration only, and may define aNURBS surface with an aspect ratio appropriate for the selected skinpreform, and may be a “live” die (not shown). A live die has movableportions that can further shape the skin preform after forming it to theinitial single-sided die. It will be appreciated that a single-sided dieis much less expensive to fabricate than an equivalent stamp-die. Infact, the single sided die may be fabricated using computer numericalcontrol (“CNC”) machining methods and similar software as was used todesign the surface structure, thus simplifying the design-toolingprocess. Similar software may then also be used to control the build-upmaterial delivery nozzle.

[0042] Cartridge heaters 36 inside the die heat both the die and theskin preform, in this case to about 450° F., to soften the skin preformand assist in the formation process. A vacuum port 38 delivers a vacuumbetween the skin preform and the die, and a clamp 40 holds the skinpreform in place and provides a nominal metal-to-metal seal 42. An inletport 44 in the clamp 40 admits gas, such as heated nitrogen, on thedistal side 46 of the skin preform at a pressure between 20-110 psi,preferably about 45 psi, and at a controlled rate for a ramp-up periodof about 20 minutes to conform the skin preform to the shape of the die.The length of the ramp depends on the desired aspect ratio. A 20 minuteramp is appropriate for an aspect ratio of 0.6 in 0.060 inch thick sheetof 6061-O, for example, while an aspect ratio of 0.1 requires only a 2minute ramp. The gas flow is controlled by a dome pilot operated gasvalve, available from TESTCOM of Elk River, Minn., for example, thevalve being computer controlled to provide the selected gas flow rate.For other skin preforms, such as a MYLAR™ skin preform, the skin isformed in as little as two minutes, and a computer-controlled valve isoptional. When the aluminum skin preform is conformed to the die, thesurface of the skin becomes slightly textured, like an orange peel. Thistexturing enhances the subsequent bonding between the skin and thebuild-up. Furthermore, the use of a gas to conform the skin preform tothe die results in a clean surface, unlike the surface that would beobtained if another fluid, such as an oil, were used to hydroform theskin. Additionally, the gas has relatively little thermal capacity, andis easily vented, thus the cool-down of the skin in preparation for thebuild-up is relatively fast.

[0043] When using other metals, such as titanium or stainless steel, forthe skin preform, an additional step of explosively setting the skinpreform to the die may be desired to overcome spring-back. An explosiveset is a rapid discharge of gas applied to the distal side of the skinpreform after the skin preform has been formed to the die by the initialgas ramp. For example, a skin preform of 3045 stainless steel 0.006thick and approximately six inches in diameter may be explosively set tothe die with a burst of heated nitrogen at reservoir pressure of 650 psithat releases 1,500 cubic inches in about 0.14 seconds to the distalside of the skin preform. The gas is transferred from the reservoir tothe skin preform via a burst-diaphragm poppet valve, also known as a“rupture valve”, available from PETER-PAUL ELECTRONICS COMPANY of NewBritain, Conn.

[0044]FIG. 3B shows the skin 48 after forming to the die 32. The skin isdry, clean, and warm, and is ready for the build-up to be applied. Anozzle 50 applies polymer to the skin in a selected pattern, asdescribed above. The nozzle has two inputs 52, 54 for a two-component (Aand B) polymer 56, 58, such as an epoxy system or astyrene-acrylonitrile/polybutadiene system, but could easily be used todispense single-component polymers, such as polyvinylchloride orpolycarbonate. Additional nozzle inputs (not shown) may be present toadd accelerators, initiators, plasticizers, or the like to the polymerbefore applying the polymer to the build-up layer. The nozzle has aheater 60 for heating some polymers to lower their viscosity. The nozzlehas an orifice 62 approximately 0.090 inches in diameter and the nozzlecan travel at speeds up to 20 feet-per-second, although these specificsare given as examples only and are not meant to limit the invention. Thenozzle has delivered polymer to build up the first zone 25 next to theskin 23 and has started to build-up a second zone 26 of honeycomb cells,which are exaggerated in height for ease of illustration.

[0045] Filler 61, such as microspheres or fibers, is supplied to thenozzle 50 by a positive-displacement pump 63, where the filler combineswith the resin. In some instances, it may be desirable to combine thefiller with the resin in an optional premixer 64, which can also be usedto mix multi-component resins or a resin and other substance, such as anaccelerator or plasticizer. In other instances, multi-component resinsmay be supplied to the nozzle, where they mix prior to being dispensedon the skin. In this instance, glass spheres are used as a filler. Thespheres are approximately 0.00015 to 0.008 inches in diameter and aresold under the name K-20 ZEEOSHPERES™ available from 3M/SpecialtyAdditives Division of Saint Paul Minn., for example. Alternatively,zirconia microballons or ceramic spheres of similar dimensions may, beused. The microspheres improve the fluidity, i.e. lower the viscosity,of the dispensed polymer. Lowering the viscosity of the applied polymerby the addition of microspheres or microballons is particularlydesirable when dispensing thermoplastic resins, which generally have ahigher viscosity than many of the thermosetting resins at similartemperatures. Lowering the viscosity of the applied resin also allowsuse of a smaller orifice on the nozzle (for given delivery pressure) formore precise dispensing of the build-up material, and also results in ahigher flow rate, thus reducing the time required to build-up a givenstructure. Glass microballons are preferred to use in conjunction withpolycarbonate polymer because of the high delivery pressures used toextrude the polycarbonate through the nozzle. These delivery pressuresare obtained by pulse-width modulation before the nozzle.

[0046] Other types of spherical fillers can be used, for example, solidaluminum microspheres will withstand high delivery pressures used inconjunction with some resins, or other types of microballons may be usedto fill other polymers, such as epoxy resins, which can be delivered ata lower nozzle pressure. Fillers may be combined for some applications,including a variety of different microspheres, or microspheres incombination with other fillers, such as fibers to improve tensilestrength, or other solids, such as clay, cellulose, or silica may beused.

[0047]FIG. 3C shows the skin 23 and the build-up 25, which have not beenremoved from the die 32 since the skin was formed to the die, and abacking blank 66 positioned for assembly to the build-up. The backingblank is a sheet of 6160-O aluminum approximately 0.010 thick, but couldbe other material, as discussed above in relation to the skin. Ingeneral, it is preferred that the material of the backing blank providetensile strength to the back of the surface structure. In this instance,in which the build-up has been patterned into a honeycomb transitioningto columns, another desirable feature of the backing blank is that it isdeformable.

[0048] As for the skin-forming process, described above, the backingblank 66 is heated to about 250° F. by hot nitrogen. The build-up 25 ismaintained at a temperature of approximately 210° F. by the cartridgeheaters 36 in the die. This temperature allows the build-up layer toattain approximately 70% of its compressive strength while remainingslightly softened, so that the backing will bond to the build-up duringthe backing-forming process. Other build-up materials may be havegreater or lesser portions of their ultimate compressive strength duringthe backing-forming process, depending on the strength of the build-upand the type of backing blank used.

[0049]FIG. 3D shows the surface structure 68 in the die 32 after thebacking 70 has been formed and bonded to the build-up 25. The backinghas been formed to the build-up by applying a burst of gas to the distalside 72 of the backing preform (see FIG. 3C). The gas was applied at apressure of approximately 30 psi. Of course, if the build-up issubstantially flat, such as is shown in FIG. 1C, the backing would nothave to deform to the structure of the build-up, but should stillconform to the surface of the build-up. In the present example, thebacking 70 acts as an encapsulating skin seals gas in the voids 74 ofthe build-up layer 25. These gas-filled voids enhance shock absorptionof the surface structure 68 and also provide thermal insulation betweenthe skin 23 and the backing 70. In some embodiments, forming a metalbacking to the build-up work-hardens, or strain-hardens, the backing toprovide additional strength.

[0050]FIG. 4A is a flow chart of a process 400 used for fabricating asurface structure according to an embodiment of the present invention. Askin preform is formed to a single-sided die by applying gas pressure(step 402). Then, without removing the skin from the die, the build-uplayer is formed on the skin (step 404). Next, the backing is conformedto the build-up layer (step 406).

[0051]FIG. 4B is a flow chart of another process 410 used forfabricating a surface structure according to another embodiment of thepresent invention. A skin preform is formed to a single-sided die with aramp of gas pressure (step 412) starting at between 20-110 psiincreasing at 2-9 psi per minute. The skin preform is then explosivelyset to the die (optional) with a burst of gas (step 414) at 45-650 psi,increasing at 45-650 psi per second. A first zone of a build-up layer isformed on the skin by applying polymer to the surface of the skin (step416). A second, patterned, zone is formed on the first zone of thebuild-up layer by applying polymer through a nozzle that is controllablymoved to create the desired pattern (step 418). Optionally, additionalzones may be applied, either by applying polymer through the nozzle, orby other means (not shown). A backing is conformed to the build-up layer(step 420), and the surface structure is removed from the die (step422).

[0052]FIG. 5A is a simplified diagram of a three-station surfacestructure fabrication apparatus 500. The apparatus has three stations502, 504, 506 and three die 503, 505, 507. After processing at eachstation, the die are rotated 508 to the next station so that the surfacestructure is formed without removing any component of it from the dieuntil the structure is completed. The first station 502 includes a firstdie 503 and cap 510 and applies gas from a gas reservoir 512 to set askin preform (not shown) to the first die. The second station 504includes a second die 505 that has a skin 514 that was formed to thesecond die when the second die was at the first station. A deliverynozzle 516 mounted on a crane arm 518 applies polymer to the skin toform a build-up layer on the skin.

[0053] The third station 506 includes a third die 507 and a cap 520. Thedie has a skin and build-up layer that were formed at the first andsecond stations. At the third station, a backing preform (shown) 6061-Oaluminum alloy is conformed to the build-up layer by applying a burst ofgas from the gas reservoir 512. A diverter valve 522 selects whether gasfrom the reservoir will be supplied to the first station 502 or thethird station 506. Alternatively, two separate gas reservoirs could beused, one for each station.

[0054] The build-up step at the second station typically requires moretime than the combined times at the first and third stations, so asingle reservoir with the diverter valve simplifies equipmentrequirements.

[0055]FIG. 5B is a simplified flow chart of a process 501 forfabricating a surface structure using a three-station line. At the firststation, the skin preform is formed to a single-sided die using gaspressure (step 503). The die is then moved to the second station (step505), where a build-up section is applied to the skin (step 507). Thedie is then moved to the third station (step 509), where the backing isapplied (step 511). The surface structure is then removed from the die(step 513), and the die is readied for another assembly sequence. In apreferred embodiment, the die is moved from one station to the next byrotating a platen supporting at least three single-sided dies. The diescan be all of the same shape, or may define different shapes.

[0056]FIG. 6 is a simplified diagram of an apparatus 600 for fabricatingsurface structures. The apparatus may be used in the three-stationapparatus described in conjunction with FIG. 5, or may be used in otherconfigurations, such as a linear assembly line or a single-die system.An operator 602 enters commands to a controller 604 through a userinterface 606. The controller could be a personal computer, asingle-board computer (“SBC”), or programmable logic control (“PLC”)module, for example. The controller includes a processor 601 and amemory 603 that contains an operating program 605 that has been enteredinto the memory.

[0057] The controller controls several functions of the apparatusaccording to the control program 605 stored in the memory 603 viacontrol lines 608, only a few of which are shown for purposes ofillustration. The controller can control the operating pressure of thegas reservoir 512 according to a feedback signal from a pressure gauge519, the operating temperatures of the gas heating element 608, nozzleheater 60, and die heaters 36, according to feedback signals fromtemperature sensors, such as thermocouples (not shown), the deliveryrate of polymer from the resin source 515 by a gas-pressure controlvalve 517, and three-dimensional positioning and travel of the nozzle50, among other functions. The controller can control the addition offiller, such as microspheres or fibers, from a filler source 632 bycontrolling the operation of a positive-displacement pump 634. Gaspressure may be applied to the die for skin forming or backingoperations through a valve 630, while a different valve 632 may be usedfor explosively setting some skin materials, as described above. The gasreservoir 512 receives process gas from a gas source 636, which could bea gas tank, gas line, or pump, such as a high-pressure pump attached toa liquid nitrogen source.

[0058] The nozzle 50 is carried on a crane arm 618. The crane arm iscontrollably movable with respect to the workpiece 620, and the positionand travel of the nozzle is controlled by motors 622, 624, 626, such asstepper motors, servo motors, hydraulic motors, or voice coil motors. Ina preferred embodiment, servo motors coupled to the crane 622, are usedin conjunction with servo motors mounted on the crane 624, and coupledto the nozzle to position and move the nozzle tangentially to thesurface of the skin, while a voice coil motor 626 positions the nozzleperpendicularly from the surface of the skin. A hydraulic ram 628 isalso used to adjust the height of the crane arm. Thus, the nozzle maytraverse a selected path, such as to define a hexagonal web, while thenozzle maintains a selected distance from the surface of the skin, evenon surfaces with multiple and complex curves.

[0059] The invention has now been explained with reference to specificembodiments; however, other various modifications, alternatives andequivalents may be used. For example, the composition of materialdispensed from the nozzle may be intentionally varied to optimizeselected parameters of the surface structure. Similarly, fillers may becombined with the resin in a pre-mixer, rather than at the nozzle. Otherembodiments will be apparent to those skilled in the art. Therefore,this application should not be limited except by the following claims.

What is claimed is:
 1. An article of manufacture comprising: a skinhaving a surface; a build-up applied to the surface of the skin, thebuild-up being bonded to the skin; and a backing conformed to and bondedto the build-up wherein the skin, the build-up, and the backing form aunitary surface structure.
 2. The article of claim 1 wherein the skincomprises metal sheet.
 3. The article of claim 2 wherein the surface ofthe metal sheet includes a curve.
 4. The article of claim 1 wherein thebuild-up comprises a polymer.
 5. The article of claim 4 wherein thebuild-up further comprises a second phase.
 6. The article of claim 5wherein the second phase comprises microspheres or microballons.
 7. Thearticle of claim 5 wherein the second phase comprises fibers.
 8. Thearticle of claim 1 wherein the build-up comprises a first zone forming asheet in contact with essentially the entire surface of the skin.
 9. Thearticle of claim 1 wherein the build-up comprises a second zone having aplurality of walls defining voids, the voids being sealed by thebacking.
 10. The article of claim 9 wherein the build-up is applied in aselected pattern.
 11. The article of claim 10 wherein the build-up isapplied in a liquid state through a nozzle.
 12. The article of claim 10wherein the build-up is applied in a powder state.
 13. The article ofclaim 10 wherein the build-up is a metal, the metal being applied to theskin in a molten state with a flame sprayer.
 14. The article of claim 9wherein each of the walls is essentially perpendicular to the surface ofthe skin.
 15. The article of claim 9 wherein a first section of thewalls defines a hexagonal pattern and a second section of the wallsdefines a columnar pattern, the walls transitioning from the hexagonalsection to the columnar section.
 16. A surface structure comprising: ametal skin having a surface, the surface including a curved portion; apolymer build-up applied to the surface in a selected pattern, thebuild-up having a first zone forming a sheet in contact with essentiallythe entire surface and a second zone having a plurality of walls, eachwall being essentially perpendicular to the surface, the walls proximalto the first zone defining a hexagonal pattern and transitioning to acolumnar pattern distal from the first zone; and a backing conformed tothe polymer build-up to seal voids between the walls and to form aunitary surface structure.
 17. An article of manufacture comprising: askin with a surface, the surface including a curved section; a build-upapplied to the skin in a selected pattern, the build-up having aplurality of cell walls, each of the plurality of cell walls beingessentially perpendicular to the surface over the curved section. 18.The article of claim 14 wherein the selected pattern is a hexagonalpattern.
 19. An article of manufacture comprising; a skin with asurface; a build-up applied to the surface in a selected pattern to forma plurality of cell walls, the cell walls defining a hexagonal sectionand a columnar section, the hexagonal section being proximate to thesurface, and the cell wall transitioning from the hexagonal section tothe columnar section.